Allow default Accept-Encoding

QNetworkRequest request("http://example.com");
request.setRawHeader("Accept-Encoding", "gzip, deflate");
QNetworkReply *nam->get(request);
connect(reply, &QNetworkReply::finished, [=]() {
    QByteArray ba = reply->readAll(); // ba contains compressed data instead of normal html page
    doSomething();
});

    value = request.headerField("accept-encoding");
    if (value.isEmpty()) {
#ifndef QT_NO_COMPRESS
        request.setHeaderField("Accept-Encoding", "gzip, deflate");
        request.d->autoDecompress = true;
#else
        // if zlib is not available set this to false always
        request.d->autoDecompress = false;
#endif
    }

    value = request.headerField("accept-encoding");
    if (value.isEmpty() || value == "gzip, deflate") {
#ifndef QT_NO_COMPRESS
        if (value.isEmpty() {
            request.setHeaderField("Accept-Encoding", "gzip, deflate");
        }
        request.d->autoDecompress = true;
#else
        // if zlib is not available set this to false always
        request.d->autoDecompress = false;
#endif
    }